This is usually the result of a grass seed blocking a salivary duct under the tongue. This is followed by fluid accumulation and inflammation with variable swelling visible under the jaw.
A grass seed awn can often be seen protruding from the duct opening under the tongue. Pus may be discharging around it. The distended salivary duct may be visible.
Major differential diagnoses include woody tongue and possibly lumpy jaw.
Examination of the mouth is necessary to diagnose and relieve the obstruction. This will require good head restraint (halter, nose grips), lighting (head lamp), and possibly forceps and a scalpel.
The grass seed can usually be massaged out to relieve the obstruction. Sometimes an incision with a scalpel is necessary to access the grass seed with a pair of small alligator forceps. No further treatment is necessary once the grass seed is removed and patency is established.
Preventative measures are unwarranted given its rarity.